2012 Atlantic Hurricane Season Birdseye Discussion #141

By: NCHurricane2009 , 12:02 PM GMT on October 24, 2012

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...WEDNESDAY OCTOBER 24 2012...8:15 AM EDT...
Tropical Storm Sandy moving northward toward Jamaica as expected...and is about to become a hurricane at any time now. Although Sandy has intensified over the last day...she has not rapidly intensified so far. Regardless...Sandy remains a significant heavy rain and wind threat to Jamaica...eastern Cuba...Haiti...and the Bahamas. See the Sandy special feature section below for additional details on this potentially severe situation. In the event Sandy begins to show signs of rapid intensification before I release my next full discussion...I will be releasing special updates.

Tropical depression nineteen has strengthened into Tropical Storm Tony in the last day. The new tropical storm is expected to stay over open waters. See the Tony special feature section for additional details.

A 1009 mb surface low WSW of the Cape Verde Islands...associated with the eastern of the two tropical waves in paragraph P4 of the tropical belt discussion...has become a little better organized under favorable upper winds as of 5 AM EDT this morning.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM SANDY...
Preparations for Sandy in Jamaica should have been completed by now with the storm expected to make landfall there later this morning. Under the guise of impact statement (b) in Figure 1...Haiti and the Dominican Republic can expect heavy rains with potential flash floods and mudslides...with the western half of Haiti getting some gusty winds as the impact swath in Figure 1 shows. Eastern Cuba and the Bahamas should finish preparations during the daylight hours of today...with conditions in these areas to deterorate later tonight and tomorrow.

Concerning the east coast of North America and Bermuda...tropical storm watches have been issued for SE Florida...but the impact info in Figure 1 suggests that SE Florida should at most see increased sea swells and rip currents on the coast. However...residents in SE Florida should be aware in case the storm hooks more leftward than expected in track. Their remains debate about the long-term solution of Sandy beyond the scope of the forecast track in Figure 1. Forecasts such as the one I present in Figure 1 are less threatening to the NE United States and Atlantic Canada...but moreso to Bermuda. The GFS and Euro models show a system hooking leftward as an intensifying non-tropical gale...with the Euro showing a strike to the NE United States...and the 00Z GFS showing a strike to Atlantic Canada.

Latest watches and warnings are found under the public advisory for Sandy at www.nhc.noaa.gov.

Track-wise for Sandy...she is tracking a bit faster to the north than I and the NHC previously thought...and she is tracking toward the east end of Jamaica more in alignment with my previous short-term solution than the NHC's previous short-term solution. Despite this...the previous NHC short-term solution will probably end up with less error than mine when evaluated because I showed a slower northward speed than the NHC's. I previosly predicted a track arcing from NNE to NNW around the curvature of an imagined upper vortex near the Isle of Youth of western Cuba. However...this feature did not quiet amplify into an upper vortex like I previously thought...so my updated forecast track in Figure 1 below shows a straight north track rather than an arcing track. This means I agree with the NHC track solution thru 11 PM Thu. 11 PM Thu continues to be coincident with when the GFS model shows the paragraph P2 SE US surface ridge N of Sandy at peak strength...so I agree with the slower NHC forecast track between 11 PM Thu and 11 PM Fri...but I am a hair faster than the NHC during this timeframe because this is also when that ridge weakens. Beyond 11 PM Fri...I continue to show an ENE track...creating a southward and more offshore bias with respect to NHC's solution...and also creating a track more threatening to Bermuda. I still agree with the ENE track because the GFS still creates a strong North America surface ridge on the convergent back side of the paragraph P1 upper troughing...and such a ridge would block progression toward North America. Instead...the system would get attracted toward the paragraph P2 deep-layered cyclone (which should be east of Sandy by the end of the 5-day forecast in Figure 1)...much like how during discussion #107 when Nadine got pulled ENE toward the ex-Isaac deep-layered cyclone. Because we had previously under-predicted Nadine's ENE acceleration in such a scenario...another reason I prefer to show my more ENE track in Figure 1.

Whether the non-tropical remnant of Sandy continues ENE out to sea...or whether she hooks leftward toward the NE US or Atlantic Canada depends on how much the North America surface ridge hugs the storm to the north...and depends on how interaction she has with the paragraph P1 upper trough. Such an interaction requires that Sandy is close enough or aligns with the paragraph P1 upper trough such that she takes maximum advantage of the eastern divergence of the upper trough and intensifies non-tropically. Cool air advection on her back side in turn amplifies the upper trough into an upper vortex that would whirl her NW. The Euro model shows a "perfect storm" scenario with a non-tropical Sandy slamming into the NE US at an awesome 938 mb...but such a perfect scenario requires such perfect alignment with the paragraph P1 upper trough. Statistics are usual against perfect storm scenarios...so I am not buying into this solution at this time.

Intensity-wise for Sandy...it looks like the aformentioned cut-off upper trough near the Isle of Youth of western Cuba may have disrupted Sandy's NW upper outflow a bit more than I thought...so she has not strengthened as quickly as I previously showed. Therefore my new intensity forecast in Figure 1 at most shows a category 2 hurricane (100 mph max winds) rather than a category 3 hurricane (115 mph max winds). Due to her currently impressive satellite apperance and excellent upper outflow over her south and east quadrants...I show a strengthening rate similar to my previous...coordinating with land disruptions from Jamaica and eastern Cuba while taking into account the updated track forecast in Figure 1. I begin weakening Sandy toward the end of the forecast...coincident with when the 00Z GFS shows upper SW shearing winds increasing ahead of the paragraph P1 upper trough. I agree with the transition to non-tropical by 120 hrs as Sandy should be well embedded within the eastern divergence region of the upper trough when using my track forecast and the 00Z GFS upper wind forecast.


Figure 1: Tropical Storm Sandy Forecast

My impact swath initialization in Figure 1 is the same size as my previous. This swath size continues to include Sandy's central-dense overcast and adjacent spiral band to the east...both features which have not changed in size since my previous update. This is where I believe Sandy will establish her worst impacts...although this swath is larger than the tropical storm wind radius shown in the 5 AM EDT NHC advisory. Satellite imagery shows that the eastern divergence of the cut-off upper trough near the Isle of Youth has expanded the moisture field hundreds of miles to the north and east of Sandy itself...which is why impact statement (b) in Figure 1 is written. Such far-reaching rain bands are less likely on the west side of Sandy due to the suppression by the cut-off upper trough near the Isle of Youth...followed by suppression from the incoming paragraph P1 upper trough. These suppressive effects are also why I maintain an east bias with my impact swath when compared to my forecast track.

...SPECIAL FEATURE...TROPICAL STORM TONY...
Although previously I was correct in showing a faster track with this tropical cyclone...my northward bias in my previous forecast did not happen...instead Tony following the path shown by the NHC track forecast. It appears I was giving too much credence to the 1018 mb ridge east of Tony shown in the above atmo chart and mentioned in paragraph P3. Although their is still an ever-so-slight left angle in Tony's current track with respect to the NHC forecast track...I am avoiding temptation for showing any northward bias this time around...especially as the previous NHC track path did very well...and with the current NHC track path a continuation of the previous. This is also fairly consistent with the 00Z GFS track forecast...which shows Tony continuing ENE while steered by the south side of the paragraph P2 cyclone.

Intensity-wise...I did better than the NHC did 24 hrs ago...with the NHC at that time showing too high of an intensity forecast...and my intensity forecast being much closer to what happened in reality. However...my weakish intensity forecast depended on my previous northward-biased track forecast which hurtled Tony into the westerly shear of the paragraph P2 upper trough. Instead...Tony has better followed the more south NHC track forecast such that he stayed closer in touch with his favorable warm core upper anticyclone...so as to why he followed my more weakish intensity forecast is more dumb luck to me than something that makes sense. At this point...the shear from the paragraph P2 upper trough will be on the increase regardless of how far north or south Tony will be...because the upper trough is nearing and shoving off Tony's favorable warm core upper anticyclone and t-storm canopy. I forecast no strengthening nor weakening as a balance between the unfavorable shear and favorable eastern divergence of the upper trough as Tony becomes non-tropical.


Figure 2: Forecast for Tropical Storm Tony

Impact swath in Figure 2 is based on the size of the t-storm canopy seen in infrared satellite...and extrapolating that along the forecast track. The impact swath is smaller than previous despite Tony being stronger than previous...because Tony has tightened up a bit while strengthening.

...MID-LATITUDES DISCUSSION...
P1...Surface frontal system in the mid-latitudes continues entering from SW Canada and the western US in the upper-left corner of the above atmo chart. In the upper-levels...the system has two impulses...with the eastern divergence of the first upper trough (located over the Great Lakes and Ohio Valley) driving a 1017 mb frontal depression moving into the NE US. Vast divergence east of the second upper trough driving a few frontal depressions across the western and central US. Low-level warm air advection ahead of the frontal depressions supports an upper ridge persisting over the central US...albeit this upper ridge is now broken by the aformentioned Ohio Valley upper trough.

P2...987 mb deep-layered cyclone/upper trough centered over SE Canada is still shifting eastward into the western Atlantic. In the last 24 hrs...the surface center has weakened from 987 to 993 mb while becoming stuck under the less-divergent upper vortex of the upper trough. In the last two days...southern portion of the upper trough has cut-off over the NW Caribbean Sea in the vicinity of the Isle of Youth of western Cuba (see Sandy special feature section for how this upper trough continues interacting with Sandy). Upper convergence on the west side of the NW Caribbean cut-off upper trough supports increasingly dry air in Gulf of Mexico. Meanwhile...1023 mb surface ridge over the SE US remains supported by western upper convergence of the deep-layered cyclone's upper trough. Warm air advection ahead of the deep-layered cyclone continues supporting an upper ridge now shifting from the central to the NE Atlantic.

P3...Upper vortex in the NE Atlantic remains amplified...thanks to cool air advection on the back side of the remnant gale of Rafael. Upper convergence on the west side of the upper vortex supports dry air and 1018 mb surface ridge transferring from the central to the eastern Atlantic. After being centered just north of the Azores in the previous discussion...the remnant surface gale of Rafael has turned on a more easterly track toward Portugal in deep-layer W flow on the south side of the upper vortex and north of the aforementioned 1018 mb ridge. The gale has weakened from 990 to 994 mb in the last 24 hours while beneath the non-divergent upper vortex.

...TROPICAL BELT DISCUSSION...
P4...Upper ridge in the eastern tropical Atlantic persists. Tropical wave W of the Cape Verde Islands in the previous discussion...and tropical wave over and south of the Cape Verde Islands in the previous discussion...are both continuing westward in the waters WSW of the islands this morning. While both tropical waves are under the favorable low shear and upper outflow of the upper ridge...so far no signs of organizing t-storm activity except for a 1009 mb low near 32.5W-10N that has very recently become a little better organized.

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